You've chosen to delete spam mails outright, this makes it difficult to find
out in hindsight what has gone wrong. Maybe you can reconfigure your system in
a way that spam mails are only tagged, and not deleted (reject/deliver), while
your mail server or client has a filtering rule that moves the mail to a
quarantine folder.

Then it is possible to look at the Received-headers, and at the X-Spam-Status headers of the blocked mail, and find out which SpamAssassin rules triggered.